In a Long Term Evolution (LTE) system defined by Third Generation Partnership Project (3GPP), an uplink data transmission normally uses a demodulation reference signal (DMRS) for channel estimation.
A base station allocates a physical resource block (PRB) or a plurality of PRBs for transmitting uplink data for each User Equipment (UE) which needs to transmit uplink data. The minimum granularity of resource mapping is referred to as a resource element (RE), and the resources for carrying the DMRS are located within a PRB that the base station allocates to the UE for transmitting uplink data. For example, a pilot frequency structure of an uplink DMRS as defined in Long Term Evolution (LTE) Rel-8 is shown in FIG. 1. A sub-frame, which is divided into two slots, consists of 14 symbols, and each slot contains 7 symbols. Within each allocated PRB, the resource element carrying the DMRS occupies the fourth and the eleventh symbols, i.e., the symbols in the middle of each slot. A DMRS sequence may be generated by cyclically shifting a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. Different CAZAC sequences generate different DMRS sequences. The base station informs a UE about the cyclic shift (CS) used for generating the DMRS sequence through a control signaling, and the UE generates the DMRS sequence in accordance with the control signaling.
In a LTE-Advance (LTE-A) system, to further increase the uplink transmission rate of a UE, uplink Multiple Input Multiple Output (MIMO) transmission is introduced. The UE may perform a multi-layer spatial multiplex transmission through precoding. To support the multi-layer transmission, the base station needs to allocate a DMRS sequence for each transmission layer of the UE. The DMRS sequences are orthogonal to each other. In order not to increase complexity to system design, the respective DMRS sequences of the transmission layers may occupy the same time-frequency resources. That is, the DMRS time-frequency pattern as defined by LTE Rel-8 is still used, and code division orthogonality is achieved by configuring different CSs for generating the DMRS sequences.
For Multi-User-MIMO (MU-MIMO) scheduling, that is, at least two UEs are paired for transmission, if the PRB resources allocated by the base station to the paired UEs are not aligned, the lengths of the DMRS sequences used by the paired UEs may be different. DMRS sequences of different lengths cannot achieve code division orthogonality merely through CS. To ensure the orthogonality between the DMRS sequences of MU-MIMO UEs with allocated non-aligned PRB resources, the LTE-A system introduces an Orthogonal Cover Code (OCC) sequence-based code division mode, i.e. besides allocating for each UE a CS for generating a DMRS sequence, an OCC sequence will also be allocated to the UE to act on the DMRS sequence in the two slots of a PRB, and the OCC sequences for the UEs are different.
The OCC sequences can be {1, 1} and {1, −1}. It is assumed that UE1 and UE2 are paired UEs scheduled for MU-MIMO, sequence {1, 1} is allocated to UE1, and sequence {1, −1} is allocated to UE2. Within the first slot of a certain PRB, UE1 and UE2 each multiplies a DMRS sequence generated after the cyclic shift with the first bit of its OCC sequence, respectively, and within the second slot, multiplies the DMRS sequence generated after the cyclic shift with the second bit of its OCC sequence, respectively. In such case, to achieve the orthogonality, the CSs used by each UE within the two slots of the PRB are the same, that is, the UE does not carry out a sequence group hopping between the slots. The OCC sequence can also be used to strengthen the orthogonality among DMRS sequences of multiple layers. In an LTE-A system, OCC sequences can also be used for DMRS orthogonality in single UE MIMO. That is, different OCC sequences can be allocated to different transmission layers of a single UE. In such case, a control signaling delivered by the base station is needed to inform the UE about the CS and the OCC sequence used for generating the respective DMRS sequences of the transmission layers.
Current R1-101008 proposal presents multiple combinational configuration items of the CS and the OCC sequence that can be applied in generation of DMRS sequences, wherein different combinational configuration items of the CS and OCC sequence can be set corresponding to different numbers of the transmission layers of a UE. The multiple combinational configuration items based on different rank values, which are presented by the R1-101008 proposal, are specifically shown in Table 1, DMRS resource configuration table.
TABLE 1DMRS Resource Configuration TableDMRSResourceRank = 1Rank = 2Rank = 3Rank = 4configurationOCCOCCOCCOCCnumberCSsequenceCSsequenceCSsequenceCSsequence00{1, 1}0, 6{1, −1}, {1, −1}0, 4, 8{1, −1}, {1, 1}, {1, −1}0, 3, 6, 9{1, −1}, {1, 1},{1, −1}, {1, 1}11{1, −1}1, 7{1, −1}, {1, −1}1, 5, 9{1, 1}, {1, −1}, {1, −1}1, 4, 7, 10{1, 1}, {1, −1},{1, 1}, {1, −1}23{1, 1}3, 9{1, −1}, {1, −1}2, 6, 10{1, −1}, {1, 1}, {1, −1}2, 5, 8, 11{1, −1}, {1, 1},{1, −1}, {1, 1}35{1, −1}5, 11{1, −1}, {1, −1}0, 4, 8{1, 1}, {1, −1}, {1, 1}0, 3, 6, 9{1, 1}, {1, 1},{1, 1}, {1, 1}46{1, 1}6, 0{1, 1}, {1, 1}1, 5, 9{1, −1}, {1, −1}, {1, −1}1, 4, 7, 10{1, −1}, {1, −1},{1, −1}, {1, −1}57{1, −1}7, 1{1, 1}, {1, 1}2, 6, 10{1, 1}, {1, 1}, {1, 1}2, 5, 8, 11{1, −1}, {1, −1},{1, −1}, {1, −1}69{1, 1}9, 3{1, 1}, {1, 1}3, 7, 11{1, −1}, {1, −1}, {1, −1}711{1, −1}11, 5{1, 1}, {1, 1}3, 7, 11{1, 1}, {1, −1}, {1, 1}
In Table 1, Rank denotes the number of transmission layers of a UE for transmitting uplink data. The example shown in Table 1, assumes that the system can support up to four transmission layers, the base station selects the combinational configurations of CS and OCC sequence allocated to the UE according to Table 1, and informs the UE of the rank values and the DMRS resource configuration number. The UE searches the preset Table 1 according to the informed rank value and the DMRS resource configuration number, and acquires the corresponding combinational configuration of the CS and OCC sequence resource for generating the DMRS sequence as allocated by the base station.
It is found that, in the current DMRS resource configuration method, the combinational configuration items of the CS and OCC sequence resources established corresponding to different rank values are independent from each other, which to some extent increases the complexity for defining system specifications.